The invention disclosed herein is a viscometer for measuring the viscosity of liquids, particularly, liquids with Newtonian or approximately Newtonian flow properties.
In capillary viscometers the test liquid is allowed to flow through an outlet tube (measuring tube), which is narrowed into a capillary tube above the outlet. There are two axially spaced apart reference marks in the tube and the space or volume between the two reference marks is precisely known. During measurement of viscosity, the sample liquid is allowed to flow through the measuring tube and the time that it takes for the liquid meniscus to drop from the upper to the lower reference mark is accurately determined. From the measured time, the viscosity of the liquid to be investigated can be calculated with the formula v=t.multidot.K where
v=kinematic viscosity (mm.sup.2 per sec.) PA1 t=measured flow time PA1 K=instrument constant
In prior art viscometers two annular reference marks on the measuring tube are used and the flow time of the liquid meniscus between the two marks was measured manually with a stop watch. Numerous other methods for observation or for sensing when the liquid meniscus passes a reference mark have also been suggested and used.
Optical sensing of the reference marks is described by Kirchner, Chem. Eng. Techn. 31, 525 (1959) and Hughes and Rohen, J.Sc. Instr. 2, 12 (1969). Detecting the meniscus at the reference marks using fiber optics has been proposed by Smith, Analyst Ang. 95, 743 (1970) and in the German GM 7 104 411. German Patent No. 832,691 discloses how to sense the meniscus using high voltage sparks. U.S. Pat. No. 3,798,960 describes a capillary viscometer into which resistances are sealed by means of resin, particularly, an epoxy resin, for sensing the meniscus.
Since the majority of liquids are clear or translucent, the meniscus is detected mostly by visual observation or photoelectric sensing. In the category of photoelectric sensing, the method using photoconductive fiber optics has prevailed. This method is used widely in automated viscosity measurement procedures. For liquids with high electrical resistance, the high voltage spark method is usually used as disclosed in German Patent No. 832,691 for measuring the viscosity of mineral oils.
These known methods have one of several disadvantages:
1. All methods employing visual observation are subject to subjective measuring errors by the measuring individual.
2. The photoelectric methods cannot be used with black and/or opaque liquids.
3. The high voltage spark method cannot be used with conductive specimens such as aqueous systems or mineral oils containing some water, or rubbed-off metallic particles or conductive lubricating oil additives.
4. Viscometers having detector elements which are sealed in with organic substances such as resins which are exposed to the sample liquids are not resistant to solvents and/or chemical solutions at the junction point.
The objective of the invention described herein is to provide a viscometer which excludes the possibility of making subjective measurement errors that can determine the viscosity of black and/or opaque liquids that can be used for conductive specimens and, above all, is resistant to any types of chemicals, with the exception of hydrofluoric acid since all surfaces exposed to the test specimen are glass. These objectives are achieved, according to the invention, with a capillary viscometer which is characterized by the fact that glass-encased electrical resistors with positive or negative temperature coefficient are fused hermetically into the viscometer at the location of the usual reference marks.
The principle involved is to mark occurrence of the meniscus by detecting a change in electric resistivity as the meniscus passes a thermally responsive resistive element. Because of the differing thermal conductivity of air and the liquid specimen, the liquid meniscus, generates a sharp change in the measured resistance as the meniscus passes the resistive elements. The concept of detecting a meniscus by detecting the resistivity change in an element such as a thermistor is disclosed in U.S. Pat. No. 3,798,960. However, in that patent the detector elements are in direct contact with the liquid whose viscosity is being measured which means that liquids which might attack the resistive elements must be excluded from making viscosity measurements with the patented viscometer.
The invention is illustrated herein in greater detail in a viscometer of the Ubbelohde type. Viscometers of this type are described in more details in German Patent No. 673,185 as well as in German Standards DIN 51 562 and U.S. Standard ASTM D 2515.